A stretched polyamide film composed of nylon 6 has been broadly used for packaging purposes due to excellent features such as toughness, flexibility and transparency. For packaging purposes, such a film is mainly used in the forms of a bag and a cover of a cup. Specifically, a bag is prepared by laminating a thermal adhesion layer, i.e. a sealant layer, composed of polyolefin or the like on the inside of the polyamide film and heat-sealing the sealed part. A cover is prepared by covering a cup composed of polyolefin or the like with the polyamide film on a flange part and heat-sealing the flange part.
When a stretched polyamide film is used for the above-described packaging purposes, high heat-sealing strength and lamination strength are required.
It is conventionally known that adhesive property is improved by laminating a layer of an amorphous resin or a highly-adherent resin on a surface layer with coating or coextrusion (for example, Patent Document 1). However, the method has problems such as blocking and complicated production steps. Further, in the improved method thereof, there is a problem that the adhesive strength between polyamide film and a highly-adherent layer as the surface layer is insufficient.
It is conventionally known that adhesive property can be improved by increasing a heat fixation temperature to disrupt the orientation in the surface layer. However, the method has problems that high temperature is required for sufficient adhesive property and a film becomes brittle due to excessive heat treatment.
It is known that when a small amount of semi-aromatic polyamide is added to aliphatic polyamide, strength for coping with cohesive failure can be improved, the adverse influence due to moisture can be minimized, and the propagation of shock by a fall can be prevented (for example, Patent Document 1). However, the method has a problem that lamination strength is insufficient.
It is known that stretching property is improved and thickness unevenness is decreased by adding a polyamide resin having a specific chemical structure (for example, Patent Document 2). However, there is no description that the method is applied for improving adhesive property in Patent Document 2 and the method has a problem that lamination strength is not sufficiently improved.
It is known that a strength for coping with prick and mechanical strength are improved by adding a stratified silicate salt to make a planar orientation coefficient to be 0.05 or more (for example, Patent Document 3). However, there is no description that lamination strength and adhesive property are improved, though there are descriptions of a stretch due to moisture absorption and lamination workability. In addition, the method has problems that a surface layer is susceptible to a cleavage and lamination strength is not sufficient.